1. Field of the Invention
The present invention relates to a superconductive magnet device to improve uniformity of a uniform magnetic field space between a pair of superconductive magnet bodies.
2. Background Art
Recently superconductive magnet devices have come to be widely used as a generating source of a static magnetic field that has a high intensity and a temporal stability. In particular, superconductive magnet devices have been popularly employed as a generating source of a static magnetic field in medical tomography equipment (magnetic resonance imaging equipment) or silicon single crystal pickup devices. Most of those conventional superconductive magnet devices were of cylindrical solenoid type, while more popularly used recently are such devices provided with two superconductive magnets horizontally or vertically opposed each other with a large clearance therebetween in order to utilize their large opening and spacious magnetic field area.
In the field of superconductive magnet devices for MRI equipment, for example, a vertically opposed type superconductive magnet device provided with a wide opening is rapidly increasing, which brings about a comfort of roominess to a patient while providing a better accessibility to the patient for an examination staff.
FIG. 18 is a perspective view showing an appearance of, for example, a vertically opposed type superconductive magnet device used in MRI equipment, and FIG. 19 is a schematic sectional drawing and FIG. 20 is a schematic circuit diagram thereof, respectively. Referring to FIGS. 18 to 20, (1) high intensity of magnetic field, (2) temporal stability of magnetic field, and (3) uniformity of magnetic field are required as a performance of magnetic field in image pickup area 1 of a superconductive magnet device for MRI.
Moreover, it is also required to reduce the amount of evaporation of liquid helium and to reduce leak magnetic field. For the purpose of attaining a high magnetic field intensity satisfying a required performance of a magnetic field of a vertically opposed type superconductive magnet device for MRI, current density of annular superconductive coils 2, 3 is increased so that an intense magnetic field is generated. Also, for the purpose of attaining temporal stability of the magnetic field, a permanent current switch 4 is employed for permanent current mode operation so that a magnetic field becomes temporally super-stable. Further, for the purpose of attaining uniformity of the magnetic field, a plurality of annular superconductive coils 2, 3 are provided so that a higher uniformity is achieved. Then the respective annular superconductive coils 2, 3 are connected in series, through which an identical current is applied. A permanent current switch 4 is connected in parallel to these annular superconductive coils 2, 3, and ON/OFF of the permanent current switch 4 is conducted by applying or interrupting a current to a heater 5 for the permanent current switch for magnetization or demagnetization, thereby a permanent current mode being achieved. Coil protecting elements 6, 7 are provided at appropriate points for protection against a high voltage generated at the time of normal conduction transition (quench) in the magnetization or demagnetization. Current leads between coils, and a part of leads for the permanent current switch 4 or the coil protecting elements 6, 7 are connected through connecting tubes 8, 9.
For the purpose of reducing evaporation amount of liquid helium, cryogenic containers 10, 11 are entirely covered with vacuum adiabatic containers 12, 13 and, further, one or two thermal shield baths (not shown) or superinsulation materials (not shown) are provided between the cryogenic containers 10, 11 and the vacuum adiabatic containers 12, 13. In addition, the annular superconductive coils 2, 3, the cryogenic containers 10, 11 and the vacuum adiabatic containers 12, 13 form superconductive magnet bodies 14, 15 as a whole. Also, the thermal shield baths are cooled by a refrigerator not shown. □ For the purpose of lowering leak magnet field, upper and lower magnetic shield plates 16, 17 made of a ferromagnetic member are provided outside the respective vacuum adiabatic containers 12, 13, and yokes of a ferromagnetic body 18, 19 are placed between the magnetic shield plates 16, 17 to secure them.
In designing a superconductive magnet device, number of units of annular superconductive coils is determined, and also dimensions, positioning, number of windings, current density, etc. of the annular superconductive coils 2, 3 are strictly determined taking into consideration a magnetic field generated in the image pickup area 1 by magnetic moment of yokes 18, 19 made of a ferromagnetic material, in such a manner that all the error magnetic field components become substantially zero. In the case of a superconductive magnet device for MRI, in general, a deviation of 1 mm in dimensions or positioning will result in an influence mounting to several tens ppm in overall error magnetic field components.
Accordingly, in designing the superconductive magnet device, the dimensions, positioning, number of windings or current density have to be determined through a strict optimization so that all the error magnetic field components become substantially zero, while it is usual that uniformity of a magnetic field has a range of several hundreds ppm when actually magnetized, because of dimension tolerance in the manufacture or magnetism existing in the employed materials, etc. Particularly in the design of a vertically split type superconductive magnet device, the uniformity tends to be inferior, as compare with the conventional cylindrical solenoid type superconductive magnet device, to such an extent that a positioning error between upper and lower superconductive magnet bodies 14, 15 is added.
For the purpose of compensating the uniformity deteriorated to over hundreds ppm, as well as circumstantial influences due to magnetism of structural steel of a room in which a superconductive magnet device is installed or that of peripheral equipments, thereby improving the uniformity in the state of practical use, iron shims in the form of fine chips have been conventionally used.
Referring to FIGS. 18 and 19, reference numerals 20, 21 are iron shim chips mounted on the gap side surface of the superconductive magnet bodies 14, 15, numerals 22, 23 are iron shim chips mounted on bore portions 14a, 15a respectively provided at the center of superconductive magnet bodies 14, 15. In mounting the iron shim chips 20 to 23, from the viewpoint of adjusting the uniformity in the image pickup area 1, a smaller amount of iron shim chips can perform a greater compensation effect in the central region of the gap side surface closer to the image pickup area 1, and in a region closer to the gap within the bore portions 14a, 15a. 
In this manner, the uniformity of magnetic field can be improved by placing a plurality of iron shim chips 20 to 23 on the gap side surface of the upper and lower superconductive magnet bodies 14, 15 and in central bore portions 14a, 15a, as well as by adjusting number of the iron shim chips 20 to 23. Practically, magnetic moment of the iron shim chips 20 to 23 and magnetic field components generated by the magnetic moment in the image pickup area 1 must be precisely analyzed in advance based on the magnetic field intensity applied to the iron shim chips 20 to 23 of the respective positions, and then compensation amount of each of the magnetic field components must be determined based on the analysis of error magnetic field components that generate a uniformity of several hundreds ppm, thereby optimizing the positions and quantity of the iron shim chips 20 to 23 to be mounted. Usually, it is difficult to attain a desired uniformity in just one execution of works, and therefore the process has to be repeated several times thus gradually improving the uniformity.
In general, intensity of magnetic field in the image pickup area 1 is obtained by the following formula (1) through a development of Legendre function. Codesγ,θ,φ in the formula are illustrated in FIG. 21.[Formula 1]                              Bz          ⁢                                           ⁢                      (                          y              ,              θ              ,              ϕ                        )                          =                              ∑                          n              =              0                        ∞                    ⁢                                    ∑                              m                =                0                            n                        ⁢                                          y                n                            ⁢                                                p                  n                  m                                ⁡                                  (                                      cos                    ⁢                                                                                   ⁢                    θ                                    )                                            ⁢                                                           ⁢                              {                                                                                                    a                        m                                            n                                        ⁢                                                                                   ⁢                                          cos                      ⁡                                              (                                                  m                          ⁢                                                                                                           ⁢                          ϕ                                                )                                                                              +                                                                                                                                         n                                                m                                            ⁢                      b                                        ⁢                                                                                   ⁢                                          sin                      ⁡                                              (                                                  m                          ⁢                                                                                                           ⁢                          ϕ                                                )                                                                                            }                                                                        (        1        )            where: anm, bnm are coefficients of components determined by the form of magnetic field.
A magnetic field is referred to as a component in the form of (m, n) values developed by Legendre function. (0, 0) component is a desired uniform magnetic field component, and all others are non-uniform error magnetic field components in the image pickup area. Among the error magnetic field components, the components being m=0, i.e. (0, n) components are collectively referred to as Z components, and those being m≠0 are collectively referred to as R components.
In this manner, after adjusting uniformity of a magnetic field by the fine chip-shaped ferromagnetic shims 20 to 23, gradient magnet field coils 24, 25 are disposed directly over the fine chip-shaped ferromagnetic shims 20, 21 between the respective superconductive magnet bodies 14, 15 as shown in FIG. 19, for generating an gradient magnet field to be applied in the form of pulsation to the image pickup area 1 at the time of picking up an image.
In the conventional superconductive magnet device of above arrangement, the plurality of iron shim chips 20 to 23 are placed in the proximity of image pickup area 1 on the gap side surface of the superconductive magnet bodies 14, 15 as well as in the central bore portions 14a, 15a for adjusting uniformity of the image pickup area 1, and the uniformity can only be gradually improved by optimizing positions and number of the iron shim chips and repeating adjusting. Therefore, a problem exists in that it is more difficult to compensate (0, n) components, i.e., Z components among the error magnetic field components than to compensate R components, and the greater the n value becomes the more difficult the adjustment of uniformity becomes.
Also, in the conventional superconductive magnet device of above arrangement, the gradient magnet field coils 24, 25 are disposed right above the iron shim chips 20, 21 between the respective superconductive magnet bodies 14, 15, after adjusting uniformity of the magnetic field by the iron shim chips 20 to 23. Therefore an error magnetic field is generated due to a minute magnetism contained in component members of the gradient magnetic field coils 24, 25 or to a slight flexure of the vacuum adiabatic containers 12, 13 caused by weight of gradient magnetic field coils 24, 25. However, as the gradient magnetic field coils 24, 25 are formed by molding together not less than 6 pieces of flat plate type coils of complicated shape, it is usually impossible to provide a number of holes on the disc surface. Further, to make the image pickup area 1 as large as possible, in other words, to have the gap between the superconductive magnet bodies 14, 15 as large as possible, the gradient magnetic field coils 24, 25 are disposed close to the fine chip-shaped ferromagnetic shims 20, 21 in such a manner as nearly in close contact with each other. As a result, it is impossible to dispose or remove the iron shim chips 20, 21 in a region covered with the gradient magnetic field coils 24, 25.
To overcome the mentioned problems, the following final adjustments have been conventionally performed under the condition of gradient magnetic field coils 24, 25 being disposed.
(1) Adjust the uniformity by superimposing a small amount of direct current on the gradient magnetic field coils 24, 25 together with the pulsating current that generates a pulsating magnetic field.
(2) Dispose iron shim chips on a directly accessible region of the surface or side faces of vacuum adiabatic containers 12, 13, without being covered with the gradient magnetic field coils 24, 25.
(3) Dispose iron shim chips on the surface of or inside of the gradient magnetic field coils 24, 25.
(4) Add current shim coils.
However, the item (1) is applicable only to compensation of (0, 1) and (1, 1) components of Legendre function development. The item (2) does not provide a significant effect since the adjustment area is far from the image pickup area 1. The item (3) can provide a superior compensation effect but largely depends on the precision of the gradient magnetic field coils. And the item (4) will brings such disadvantages that the current shim coils and power supply device will be more costly, and moreover the image pickup area 1 will be narrower since the current shim coils are additionally placed.